1. Field of the Invention
The present invention relates to a device and a method for accurately controlling the flow of fluids through fluid delivery systems. More particularly, the present invention relates to a system and a method for controlling the rate of fluid flow over a wide dynamic range, relying in part upon extremely accurate measurement of the fluid removed from a source and delivered to a fluid sink.
2. Description of the Prior Art
In previous art, fixed fluid bolus volumes are transferred from a medical fluid source to a patient IV line via a cassette with inlet and outlet valves and an intermediate fluid chamber. In some systems the chamber volume is driven to vary abruptly over a fixed volume stroke, e.g., by positive and negative pressure pulses communicated across a flexible diaphragm (e.g., Siekmann, U.S. Pat. No. 4,303,376), delivering fixed boluses. If the stroke frequency is high enough, relative continuity of the output is achieved, but dynamic range of flow control is limited. At low rates, the interval between strokes becomes, excessive and flow continuity is lost, and at high rates, a maximum practical operating frequency is reached. Better continuity of flow is achieved, e.g., in a system using an elastic "piston" consisting of plastic panels on "living" hinges, driven by a cam so that the intake stroke is fast and followed by a slow, steady discharge stroke. In this example, the price for flow continuity is greater mechanical complexity.
Looking toward greater simplicity, it is advantageous to use the energy present in the pressurization of a source fluid to deliver that fluid to a patient, e.g., as in the ReadyMED elastomer fluid reservoir. (ReadyMED is a registered trademark of IMED Corporation.) The disadvantage here is a stringent requirement for constant source pressure, implying high manufacturing cost, coupled with the inflexibility of a single delivery rate for a given device. Greater economy in the fluid source is achieved, e.g., with a minibag emptied by pressurized gas, but pressure in such a source varies widely during delivery. Similar comments apply to spring-loaded reservoirs, An active flow regulator could potentially compensate for source pressure variations, making more economical fluid pressure sources usable, while at the same time adding the flexibility of programmable flow rate. Seeking such a flow controller solution in the art, there are examples of active valve control (e.g. Peter, U.S. Pat. No. 5,049,141 or Idriss, U.S. Pat. No. 4,838,887) in systems where an elastic reservoir between active inlet and outlet control valves is alternately completely filled and completely emptied as the valves are alternately opened, thus achieving fluid boluses of known volume at a controllable bolus frequency. As with the push-pull pumping systems, fixed bolus size limits the dynamic range of infusion rate of the device, whereby low flow rates result in excessive time intervals between boluses and high flow rates result in high valve actuation frequencies and resulting excessive noise and power consumption.